In general, the Czochralski method (CZ method) has been widely used as a method of manufacturing a silicon single crystal for manufacturing a semiconductor. In the CZ method, as shown in FIGS. 1 and 2, a seed crystal 102, which is a single crystal, is dipped into silicon melt 101 of polycrystalline silicon in a crucible 100 made of vitreous silica. At this time, the seed crystal is subjected to a drastic heat shock, and dislocation occurs at the tip of the seed crystal. The dislocation is removed by forming a neck 103 so that the subsequently grown silicon is not affected adversely by the dislocation. Then, the seed crystal is gradually pulled up while being rotated and the pulling speed and the melt temperature are controlled, to increase the diameter of the seed crystal to form a shoulder 104. After a desired diameter is achieved, the pulling is continued under the control that the constant diameter is maintained, to form a straight body 105. Finally, a tail 106 is formed by gradually reducing the diameter, to manufacture an ingot 107 of a silicon single crystal.
In general, as shown in FIG. 1, the vitreous silica crucible used for pulling of a silicon single crystal uses natural fused silica 108 on the outer portion to increase the mechanical strength of the crucible, and uses synthetic fused silica 109 on the inner portion to avoid mixing of the impurities.
Here, “natural fused silica” is vitreous silica made of natural silica powder, and “synthetic fused silica” is vitreous silica made of synthetic silica powder. In general, the reaction SiO2(solid)→Si (liquid)+2O occurs on the interface between the synthetic fused silica 109 and the silicon melt 101 to dissolve the synthetic fused silica 109.
When pulling a silicon single crystal, the reaction Si (liquid)+O→SiO (gas) occurs to produce SiO gas depending on the increase of the pulling temperature and decrease of the ambient pressure. In this case, as shown in FIGS. 3(a) and 3(b), the silicon melt 101 may be repelled from the surface of the synthetic fused silica 109 to generate melt surface vibration. In FIGS. 3(a) and 3(b), the melt surface vibration is exaggerated to clearly explain the state of the melt surface vibration.
When such melt surface vibration is generated, the seed crystal 102 cannot be contacted with a flat melt surface, and silicon can be polycrystallized during pulling, which is problematic. In particular, the dipping of the seed crystal and the shoulder formation, which are initial processes in pulling of a silicon single crystal, are likely to be affected adversely by melt surface vibration. The influence largely determines the quality of the pulled silicon single crystal ingot. Therefore, a technique to suppress the melt surface vibration of silicon melt in these processes has been demanded.
Patent Document 1 discloses a technique to suppress melt surface vibration of silicon melt filled in a vitreous silica crucible by adjusting the bubble content, to a certain range, of the inner surface layer of the crucible in a region near the melt surface at the beginning of pulling. This technique is based on the finding that the melt surface vibration of the silicon melt at the beginning of pulling is influenced by the bubble content of the inner surface layer of the crucible in a region near the melt surface.
For example, when a large amount of bubbles is contained in the vitreous silica crucible, the vitreous silica is dissolved as the reaction SiO2 (solid)→Si (liquid)+2O proceeds, and thus open bubbles come out as shown in FIG. 4. These open bubbles 201 suppress the melt surface vibration in the same principle as that of boiling stone which suppresses bumping.